Electrical drive systems using electrical motors to power the end-use function are in wide use for propulsion, machine drives, conveyor lines, chemical processing, material handling applications and the like. A very small sampling of exemplary drive systems appears in U.S. Pat. Nos. 3,845,366 (Metzler et al.) and 4,177,238 (Binner).
Certain types of material handling machines incorporate electric motor drive systems for moving the machine from location to location, for moving a machine "substructure" on the machine itself and for moving loads of the type the machine is designed to handle. An example of such a material handling machine is an overhead travelling crane (OTC) used in factories, steel handling bays and the like for lifting, moving and placing loads.
Such a crane traverses along a pair of elevated main rails which are parallel and spaced apart, usually by several yards. A pair of crane bridge girders extends between the rails and there are driven wheels mounted at either end of the girders for supporting the crane atop the rails. And the girders themselves have rails on them.
A substructure called a "trolley" is mounted on the girder rails and traverses the width of the bridge under motive power. A load hoist is mounted on the trolley and includes a powered hoist/lower "rope drum" or drums about which steel cable is spirally wrapped. The cable is connected to a load-lifting hook, sling, bucket, magnet or the like. With the foregoing arrangement, the operator (who usually rides in a cab which is attached to and moves with the bridge) can pick up, move and place a load anywhere in the area travelled by the crane. Other, somewhat less common operating options include radio-controlled cranes operable from the ground or other remote location and operator cabs which are trolley, rather than bridge, mounted.
An exemplary overhead crane employs two electric-motor traverse drive systems, one each for the bridge and trolley traverse drives. A third electric-motor drive system is used for hoisting and lowering loads. Such drive systems may be powered by direct current (DC) or alternating current (AC). While DC drive systems were almost universally used in older steel mills and the like, AC variable frequency drive systems are becoming increasingly common, at least in part because of the advantages of precision control and design flexibility which they offer.
In a variable frequency drive system, motor speed is a function of the frequency of the electrical voltage applied to it. Examples of AC variable frequency drive systems (used for hoist drives) are described in U.S. Pat. Nos. 4,965,847 (Jurkowski et al.) and 5,077,508 (Wycoff et al.). The leading manufacturer of overhead cranes and AC drive systems therefor is Harnischfeger Industries, Inc. of Milwaukee, Wis. One such AC drive system is sold under the trademark SMARTORQUE.RTM. and the invention involves a modification of a known type of SMARTORQUE.RTM. controller.
Hoist, bridge and trolley drives are operated by an electrical controller coupled to an operator-manipulated master switch in the cab. Such master switch has a handle with a neutral position and a continuum of positions in each of two directions from neutral. The handle thus controls drive speed in either of two directions. And, subject to the limitation described below, the farther the handle is moved away from the neutral position, the faster the drive moves the load, e.g., the bridge or trolley or the load suspended from the hoist. And the counterpart is that as a master switch is moved toward its neutral position, the drive moves the load more slowly.
In either event, the electric motor and controller "ramp" the speed change so that such change occurs no more rapidly than the maximum predetermined rate set by the slope of the ramp. The quoted expression derives its name from the fact that when depicted on a two-axis graph, the lines representing rates of acceleration and deceleration are ramp-like in shape. Usually, the control manufacturer sets such rates--they are not changed in day-to-day crane operations.
Before setting forth additional background information, an understanding of some fundamentals will be helpful. One such fundamental relates to alternating current (AC) motors and to some of the operating characteristics of a particular type of AC motor, i.e., a squirrel cage motor. Another involves some operating principles of a type of motor controller known as an adjustable frequency inverter and the way such a controller is used with an AC squirrel cage motor. Yet another involves what is known as an asymmetrical load, i.e., a load which resists motor rotation in one direction and aids such rotation in the other, and how such a load affects the motor and the control scheme when a squirrel cage motor and adjustable frequency inverter are used to power the hoist drive.
A squirrel cage motor is so named because portions of its rotor (formed with parallel conductors shorted together at their ends) resembles a squirrel cage in shape. In three phase configuration (the type used on crane drives), the motor has only three stator terminals. In other words, there are no rotor terminals "brought out" as with a wound rotor motor.
Another fundamental relating to squirrel cage motors is that the rotational speed is, in general, a function of the frequency of the applied voltage. For example, a motor having a running speed at rated output torque and 60 Hz applied voltage of about 1760 RPM would have a running speed of about 860 RPM at 30 Hz applied voltage. In recognition of this characteristic of a squirrel cage motor, the above-noted SMARTORQUE.RTM. AC drive system and other systems like it are called "inverters" and are configured to provide an output frequency (and voltage) which can be varied by changing the position of the master switch handle.
An electric motor drive system such as a crane hoist drive represents a somewhat unusual application. Unlike the bridge and trolley drives (and unlike many other types of drives not involving overhead travelling cranes), loads handled by the hoist drive are said to be asymmetrical. That is, the weight of the load either aids or resists motor rotation, depending upon the direction of load movement.
More specifically, when the load is being hoisted, the force of gravity resists such upward movement and thus resists motor rotation. On the other hand, when the load is being lowered, the force of gravity (acting, of course, in a downward direction) aids motor rotation and acts in a way to urge the motor to run faster. Loads of this type are sometimes referred to as "overhauling" loads.
A crane hoist drive is not the only type of drive called upon to handle asymmetrical loads. Any drive moving a load between two elevations, e.g., up and down a ramp or by a reversing, sloping conveyor represents such a drive.
U.S. Pat. Nos. 4,965,847 (Jurkowski et al.) and 5,077,508 (Wycoff et al.) depict examples of electric motor drive systems for use on overhead travelling cranes and, more specifically, for use on the hoist systems of such cranes. Such systems power the motor by maintaining a substantially constant ratio between the motor applied voltage and the frequency of such voltage. As a result, the motor has a substantially constant stator current and, consequently, exhibits substantially constant torque over its entire speed range.
In a crane hoist system, it is frequently required that the load be held suspended in mid-air while the crane travels to a different location. And such load holding is while the electrical hoist drive is off. Therefore, such systems are usually equipped with an electromagnetic clamping-shoe type brake which is released by energizing a large magnet coil on the brake. The brake shoes, which are spring-biased to a closed, "brake-on" position, clamp against a brake wheel when the magnet coil is de-energized. In crane hoist systems, the brake is sized and adjusted to provide braking or clamping torque on the order of about 150% of the rated output torque of the motor.
While systems of the foregoing type have been generally satisfactory, some are characterized by certain disadvantages. Electromagnetic brakes can and do fail, either partially or completely, often because of inadequate or improper maintenance. For example, the brake shoes may be permitted to wear unduly and the torque available from the brake is diminished, perhaps dramatically so. As another example, the brake may somehow become wedged open so that its shoes cannot clamp the brake wheel when the magnet coil is de-energized.
If the condition of the brake is such that it is inadequate to hold a suspended load, such load will descend, perhaps unexpectedly, when the hoist drive is turned off. That is, the operator will not be in control of such descending load and, unless alert for telltale signs of brake deficiency, may not be aware that the brake is other than fully operative. A method for checking brake load-holding torque which helps prevent uncontrolled load lowering would be an important advance in the art.